1. Field of the Invention
The present invention relates to an Optical Network Terminal (ONT) management control protocol of a Gigabit-capable passive optical network (hereinafter, referred to as GPON).
2. Description of the Related Art
In order to construct a subscriber network spanning from a telephone office to a building or a house, there have recently been proposed various network structures and progress schemes. For instance, a x-digital subscriber line (XDSL), a hybrid fiber coax (HFC), a fiber to the building (FTTB), a fiber to the curb (FTTC), or a fiber to the home (FTTH) have been proposed. From among these structures, an FTTx (x represents B, C, or H) structure may be classified into an active FTTx structure which is realized by construction of an active optical network (hereinafter, referred to as AON) and a passive FTTx structure which is realized by construction of a passive optical network (hereinafter, referred to as a PON).
A PON, which is employed for realizing a passive FTTx, has a point-to-multipoint topology by passive elements, and it is proposed as a scheme for realizing an economic subscriber network. In other words, in the PON, one optical line termination (hereinafter, referred to as an OLT) is connected to a plurality of optical network units (hereinafter, referred to as ONUs) by means of an optical distribution network (hereinafter, referred to as an ODN) of 1×N, so that a distributed topology with a tree structure is formed.
A first type of PON developed and standardized was an asynchronous transfer mode passive optical network (hereinafter, referred to as an ATM-PON), and the standardization contents are written in the ITU-T G.982, ITU-T G.983.1 and ITU-T G983.3 drawn up by the international telecommunication union's telecommunication section (ITU-T). At present, a GPON standardization is being developed by the ITU-T.
FIG. 1 is a block diagram of a conventional PON. Generally, the PON includes one OLT and a plurality of ONUs. FIG. 1 shows an example in which one OLT 10 is connected to three ONUs 12a to 12c through an ODN 16.
Referring to FIG. 1, the OLT 10 is located at a root of tree structure. More importantly, the OLT plays a central role in providing information to subscribers in an access network. This OLT 10 is connected to the ODN 16. Herein, the ODN 16 has a tree topology structure and distributes a downstream data frame, which is transmitted from the OLT 10, to the ONUs 12a to 12c. Furthermore, the ODN 16 multiplexes upstream data frames from the ONUs 12a to 12c and transmits the multiplexed frame to the OLT 10. Also, the ONUs 12a to 12c receives the downstream data frame, provides it to end users 14a to 14c, and then transmits data output from the end users 14a to 14c to the OLT 10 through the ODN 16 as upstream data frames. Herein, each of the end users 14a to 14c connected to the each of the ONUs 12a to 12c represents various types of terminating equipment of a subscriber network that can be used in a PON including network terminals (NTs).
Generally, in an ATM-PON, there are ATM cells, each of which has a size of 53 bytes, that are transmitted upward or downward in the form of frames each consisting of a predetermined number of ATM cells. In a tree type PON structure as shown in FIG. 1, the OLT 10 properly inserts downstream cells in a downstream frame, and then the downstream cells are distributed to each of the ONUs 12a to 12c. 
Furthermore, in the case of upstream transmission, the OLT 10 accesses data are transmitted from the ONUs 12a to 12c by means of a time division multiplexing (TDM) method. The ODN 16, which is connected between the OLT 10 and the ONUs 12a to 12c, is a passive device, and the OLT 10 prevents data from colliding in the ODN 16 by means of a virtual distance correction algorithm called “ranging”.
Additionally, in order to maintain security when downstream data transmission is performed from the OLT 10 to the ONUs 12a to 12c, the OLT 10 and the ONUs 12a to 12c can send or receive from each other a code key for an encryption and an operations, administration and maintenance (OAM) message for maintenance. For such purposes, each frame in upstream or downstream transmission has a corresponding data field arranged in a dedicated ATM cell or a general ATM cell, by which a message can be sent or received at predetermined intervals.
FIG. 2 is a view showing a protocol stack structure of a conventional GPON. Referring to FIG. 2, the protocol stack of the GPON includes a protocol layer 100 which interfaces with an upper layer, a GTC layer 200, and a GPON physical media dependent (GPM) layer 300. The protocol layer 100 comprises an ATM client 110, an ONT management control interface (hereinafter, referred to as an OMCI) 120, a GEM client 130, and a physical layer operation administration maintenance (PLOAM) module 140.
In the GPON protocol having this conventional structure, the GTC layer 200 converts upper frames into a GTC frame and then transmits the frame. Herein, the ATM client 110 supports a transmission method of an ATM mode, and the GEM client 130 supports a transmission method of an GEM mode.
The ATM client 110 fits transmission data to the GTC frame by the unit of a cell with a fixed length. In this case, if there is a portion that has a length shorter than a length of a cell (typically, 53 bytes) in the GTC frame, the ATM client 110 then fits the transmission data into the next frame and then transmits the frame. Accordingly, a cell is not divided in the ATM mode.
However, since a GEM frame is a packet that can have various lengths, when the GEM client 130 fits the GEM frame to the GTC frame, the GEM client 130 may divide the GEM frame and then transmits the divided frame, for efficient use of bandwidths.
For example, when the GEM client 130 receives user data from an upper layer, the GEM client 130 receives information (e.g. length) about the GTC frame, which is standing by, from the GTC layer 200, divides the user data on the basis of the information, and generates a plurality of GEM frames. Otherwise, the GEM client 130 does not divide the user data, generates one GEM frame, and then transmits the GEM frame to the GTC layer 200. The GTC layer 200 then fits the GEM frame to a GTC frame that is standing by and transmits the frame.
Also, the reception side reassembles the divided GEM frame and then transmits the frame to an upper layer.
In the prior art, there is an ONT management control protocol for a BPON. The BPON operates on the basis of an ATM as defined by the G.983.1. Further, the G.983.2 defines an ONT management control interface of the BPON. This BPON also operates on the basis of the ATM. The G.983.2 defines an ATM cell-based frame structure which can transmit the management control information.
FIG. 3. is a view showing a structure of a frame transmitting ONT management control information of a BPON defined by the G.983.2. Hereinafter, the frame constructed by fields will be in detail described.
First, an ATM header 301 represents a virtual path identifier (VPI) and a virtual channel identifier (VCI). both which are channel addresses for management control.
Then, there is a transaction correlation identifier 302 that represents a relationship between a request message and a response message.
Further, a message type 303 represents the types of messages that can be sent/received.
Further, a device identifier 304 represents a system (OxOA) based on the ITU-T G.983.1
Further, a message identifier 305 represents a managed entity and a managed entity instance.
Further, a message contents 306 represents the contents of a message defined by the message type 303.
FIG. 4. is a view showing a structure of a frame in a conventional GEM mode supporting a TDM and Ethernet service, in which only a structure for a TDM frame or an Ethernet frame is defined.
Referring to FIG. 4, the GEM frame generally includes a payload length identifier (hereinafter, referred to as a PLI)(L) (16 bits) 410, a Port ID (12 bits) 420, a frag (two bits) 430, a FFS (two bits) 440, a HEC (16 bits) 450, and a fragment payload (L byte) 460. Herein, the PLI 410, the Port ID 420, the frag 430, the FFS 440, and the HEC 450 are included in a GEM header.
The PLI 410 is a field for representing a length of a payload, and the Port ID 420 is a field for dividing traffic in order to provide a traffic multiplexing. Further, the Frag 430 is a field for representing division state of a payload, and the FFS 440 is a field for detecting and correcting a header error. The FFS 440 is an undecided field.
In the prior art, the two bits of the Frag 430 from among the GEM frame header is used to determine whether or not a currently transmitted GEM payload is a divided frame. For instance, the Frag 430 in an undivided GEM frame may be set as “11”, the Frag 430 in a start frame of a divided GEM frame may be set as “10”, the Frag 430 in a medium frame of the divided GEM frame may be set as “00”, and the Frag 430 in a last frame of the divided GEM frame may be set as “01”, so that, whether or not a predetermined GEM frame is a divided frame, or one of the divided frames which corresponds to the predetermined GEM frame can be indicated.
Also, in a GPON which is being standardized by the ITU-T, one OLT communicates with a plurality of ONT (ONU). Herein, the OLT supplies both an ATM mode and a GEM mode, and it selectively can transmit. However, it has been defined that the ONT (ONU) supplies only one mode. Further, the OLT needs an ONT management control interface to manage the ONTs (ONUs) connected to the OLT for communication. Herein, since the OLT and the ONTs (ONUs) communicate with each other by means of different modes, the ONT management control interface must supply different modes.
A protocol defined by the ONT management control interface, which has been defined by the existing G.983.2, operates on the basis of an ATM. Accordingly, an ONT management control packet is transmitted by means of an ATM cell (reference to FIG. 3) with a length of 53 bytes. Herein, VPI/VCI values, which are channel addresses for the ONT management control interface, are shown in the ATM header. Further, a cell payload transmits information such as the transaction correlation identifier 302, the message type 303, the device identifier 304, the message identifier 305, the message contents 306, and the AAL5 trailer 307. As described above, the ONT management control interface defined by the existing G.983.2 can be just applied to the ONT (ONU) operated by an ATM mode.
However, the GPON is defined by and classified into an ONT management control interface of the GEM mode and an ONT management control interface of the ATM mode. Accordingly, the ONT management control interface of the ATM mode cannot be applied to the ONT (ONU) operated by an GEM mode.
As described above, a way capable of transmitting the ONT management control interface has not been considered in the GEM protocol. Accordingly, the GEM protocol is required to have a definition about a mechanism which can transmit management control interface information.
In general, there are two conventional methods for transmitting the OMCI information in a GPON supporting both a GEM payload and an ATM payload. One method is an in-band method in which a field is always assigned to a channel assigned to a header of a frame, etc. The other method is an out-of-band method in which a frame for the OMCI information is defined.
In the out-of-band method, the current G.983.2 intact can be used for the ATM mode, but an OMCI for a GEM mode must be newly defined.
In contrast, the in-band method has a limitation in a definition of the OMCI for the GEM mode, in that the OMCI of the GEM mode must use the G.983.2 standard intact or an OMCI field having the same size as that of an OMCI of the G.983.2 in order to use the current G.983.2, because the GPON supports both the GEM mode and the ATM mode. However, the in-band method has advantages not only because of always assigning a fixed channel, but also because of the compatibility with the standard because it does not define a new OMCI.